This doctoral thesis explores the European Union’s (EU) maritime security, which is recognised as a crucial European security area and a vibrant field within current academic discourse in Irish and European contexts. This first part of this thesis is divided into three different chapters. Firstly, Chapter 1 examines the academic, institutional, and industry definitions of maritime security and maritime security operations, before providing its own definition on the latter concept, and provides a critical review of traditional and contemporary scholarly works on maritime security, naval strategy, and maritime strategy, particularly since the topic made it to the headlines globally in 2008 off the coast of Somalia. Secondly, Chapter 2 provides a historical analysis of the EU’s common foreign, security, and defence policy from the failed project of the European Defence Community in 1954 to the 2009 Lisbon Treaty, and analyses the various actors involved in the decision-making of European security and defence policies. Thirdly, Chapter 3 describes the decision-making process of the CSDP’s military missions, examines the EU’s strategic thinking in the realm of maritime security since the 2003 European Security Strategy until the 2023 second edition of the Maritime Security Strategy and, ultimately, provides an overview of the four naval missions launched, as of June 2025, in the framework of the CSDP which will be explored in depth in the subsequent case studies. The development of the EU’s maritime security, from the inaugural 2008 operation "Atalanta" to the most recent, "Aspides", is at the core of this research, and its analysis was conducted through a comprehensive multi-methods approach, investigating maritime security governance. The second part of the thesis analyses twenty different case studies across six chapters. Chapter 4 presents an overview of security and defence policies and maritime security operations of a number of Northern European countries, while Chapter 5 explores a peculiar case study in the same region, Ireland. Subsequently, Chapter 6 examines Germany in the Central European geographical area, while Chapter 7 moves to the Eastern Mediterranean Sea with the analysis of Croatia, the Republic of Cyprus, Greece, and Turkey. Then, Chapter 8 focuses on the two Central Mediterranean countries, Italy and Malta, and Chapter 9 investigates two Western Mediterranean states, France and Spain. The “Epilogue” follows in Chapter 10 with Operation Prosperity Guardian and Operation Aspides, before the thesis draws together the relevant conclusions in the final Chapter. To conduct this investigation, documents, articles, official sources, and national strategies in seventeen languages were analysed, together with forty-six elite interviews with military personnel, policymakers, diplomats, and civil servants. This has entailed historical, geopolitical, and strategic analysis aimed at understanding the rationales behind EU maritime operations, their conduct, and the strategic postures of the various EU member states. The research concludes that geography, spatial proximity to threats, and the availability of resources were, as foreseen, critical in shaping the strategic priorities of most member states. However, strategic cultures and ideological stances, like military neutrality, played an even more important role in revealing how national interests aligned or, even more importantly, diverged within the EU.